Drilling fluid, also known as “mud”, is used in the drilling of subterranean wells, and assists in the drilling operation in several ways, including by removing rock cuttings from the wellbore to the surface, by cooling and lubricating the drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts.
Drilling fluids can be formulated in a number of ways known in the art. A drilling fluid typically comprises water and/or oil or synthetic oil or other synthetic material or synthetic fluid as a base fluid, with solids in suspension. A non-aqueous based drilling fluid typically contains oil or synthetic fluid as a continuous phase and may also contain water dispersed in the continuous phase by emulsification so that there is no distinct aqueous layer in the fluid. Such a dispersion is generally referred to as an invert emulsion, water-in-oil emulsion, or oil based mud (OBM).
The density of the drilling mud is closely maintained in order to control the hydrostatic pressure that the mud exerts at the bottom of the well. It is common for a drilling fluid to include a weighting material, or weighting agent, to increase the density of the drilling fluid. Barite is the most common weighting material, although others, including hematite, are used.
Particulates in a suspension, such as a weighting material in a drilling fluid, are prone to a type of settling referred to as “sag” or “barite sag”, in which barite particles accumulate and cause variation in density throughout the drilling fluid. Barite sag can occur at dynamic conditions at low levels of shear rates, for example, during drill pipe rotation, or when running casing or logging tools.
The large density variations created by sag can create wellbore management problems, and can even result in wellbore failure. Additionally, fluid sag can lead to sticking of drill pipe, difficulty in re-initiating and/or maintaining proper circulation of the fluid, possible loss of circulation and disproportionate removal from the well of lighter components of the fluid. Barite sag is of particular concern in oil-based drilling fluids that are typically used in moderate and high pressure/temperature environment. Barite sag is also of concern in highly deviated, directional and ERD (extended reach drilling) wells, and experiments have shown that the greatest influences of barite sag occur at well bore inclinations from 40° to 60°.
Predicting and controlling barite sag has been difficult, as the influence of fluid rheology on dynamic sag is not quantitatively established. DHAST or Dynamic High Angle Sag Tester by FANN Instrument company, which is disclosed in the patent U.S. Pat. No. 6,584,833 to Jamison and Murphy, which is incorporated by reference herein, is an instrument that can measure the rate of particle settling to indicate the sag rate; however, this device has the disadvantage that it must be used in a laboratory setting and cannot be used in the field. Further, the DHAST method requires labor of about 2 man-hours per test and the test runs for a period of 15-18 hours.
Methods of predicting sag in the field have included variations of a viscometer sag test, in which drilling fluid is sheared inside a heat cup or well, and is subsequently analyzed for changes in density. In such tests, sag tendency is considered to be proportional to the change in density, but such tests do not provide a quantitative measure of the dynamic sag rate.
Thus, a need exists for a quantitative method of predicting sag based on available rheological properties of the fluid.